Diagnosing Tomato Late Blight: What You Need to Know

As a gardener, there's nothing more frustrating than discovering that your prized tomatoes or potatoes are infected with late blight. This destructive disease is caused by a fungus-like pathogen that spreads through splashing rain or wind currents, and can quickly spread to an entire field if left unchecked.

Late blight was responsible for the Irish potato famine in the mid-19th century, and it's now been detected on tomato plants in our state. To prevent further spread of the disease, the Massachusetts Department of Agricultural Resources (MDAR) is urging anyone growing tomatoes or potatoes to monitor their plants for signs of late blight.

So, how do you diagnose late blight? The symptoms are quite distinct. Small olive green or brown lesions will appear on the upper surface of the foliage or stems, and these lesions can eventually turn black as the disease progresses. Under moist conditions, a white, fuzzy growth may also appear on the underside of the leaves.

If you suspect that your plants have late blight, it's essential to act quickly. Commercial growers should begin spraying fungicide immediately, even before symptoms are spotted, while gardeners who suspect infection should dig up and dispose of the affected plants in the trash.

Spraying with a product containing chlorothalonil, a state restricted fungicide that requires certification to use, is crucial for controlling late blight. Regular spraying must continue to prevent the disease from spreading further.

For more information on diagnosing and controlling late blight, including diagnostic images and fact sheets, visit the following websites: UMass Extension, Cornell University Extension, and the MDAR Plant Pest Hotline. By being aware of the signs of late blight and taking prompt action, you can help prevent this destructive disease from spreading and protect your crops.

Diagnosing tomato late blight